Effects of linker torsional constraints on the rate of ground-state hole transfer in porphyrin dyads

Article abstract of DOI:10.1021/ic301613k

Understanding hole/electron-transfer processes among interacting constituents of multicomponent molecular architectures is central to the fields of artificial photosynthesis and molecular electronics. Herein, we utilize a recently demonstrated ²⁰³Tl²⁰⁵Tl hyperfine "clocking" strategy to probe the rate of hole/electron transfer in the monocations of a series of three thallium-chelated porphyrin dyads, designated Tl₂-U, Tl₂-M, and Tl₂-B, that are linked via diarylethynes wherein the number of ortho-dimethyl substituents on the aryl group of the linker systematically increases (none, one, and two, respectively). Variable-temperature (160-340 K) EPR studies on the monocations of the three dyads were used to examine the thermal activation behavior of the hole/electron-transfer process and reveal the following: (1) Hole/electron transfer at room temperature (295 K) slows as torsional constraints are added to the diarylethyne linker [k(Tl₂-U) > k(Tl₂-M) > k(Tl₂-B)], with rate constants that correspond to time constants in the 2-5 ns regime. (2) As the temperature decreases, the hole/electron-transfer rates for the monocations of the three types of dyads converge and then cross over. At the lowest temperatures examined (160-170 K), the trend in the hole/electron-transfer rates is essentially reversed [k(Tl₂-B) > k(Tl₂-M)k(Tl₂-U)]. The trends in the temperature dependence of hole/electron-transfer among the three dyads are consistent with torsional motions of the aryl rings of the linker providing for thermal activation of the process at higher temperatures in the case of the less torsionally constrained dyads, Tl₂-U and Tl₂-M. In the case of the most torsionally constrained dyad, Tl₂-B, the hole/electron-transfer process is activationless at all temperatures studied. The reversal in rates of hole/electron transfer among the three dyads at low temperature is qualitatively explained by the results of density functional theory calculations, which predict that static electronic factors could dominate the hole/electron-transfer process when torsional dynamics are thermally diminished.

Full text of DOI:10.1021/ic301613k

Article
pubs.acs.org/IC
Effects of Linker Torsional Constraints on the Rate of Ground-State
Hole Transfer in Porphyrin Dyads
Christopher J. Hondros,† Kunche Aravindu,‡ James R. Diers,† Dewey Holten,*,§ Jonathan S. Lindsey,*,‡
and David F. Bocian*,†
†Department of Chemistry, University of California Riverside, Riverside, California 92521-0403, United States
‡Department of Chemistry, North Carolina State University, Raleigh, North Carolina 27695-8204, United States
§Department of Chemistry, Washington University, St. Louis, Missouri 63130-4889, United States
S
* Supporting Information
ABSTRACT: Understanding hole/electron-transfer processes among inter-
acting constituents of multicomponent molecular architectures is central to
the fields of artificial photosynthesis and molecular electronics. Herein, we
utilize a recently demonstrated 203Tl/205Tl hyperfine “clocking” strategy to
probe the rate of hole/electron transfer in the monocations of a series of
three thallium-chelated porphyrin dyads, designated Tl2-U, Tl2-M, and Tl2-
B, that are linked via diarylethynes wherein the number of ortho-dimethyl
substituents on the aryl group of the linker systematically increases (none,
one, and two, respectively). Variable-temperature (160−340 K) EPR studies on the monocations of the three dyads were used to
examine the thermal activation behavior of the hole/electron-transfer process and reveal the following: (1) Hole/electron
transfer at room temperature (295 K) slows as torsional constraints are added to the diarylethyne linker [k(Tl2-U) > k(Tl2-M) >
k(Tl2-B)], with rate constants that correspond to time constants in the 2−5 ns regime. (2) As the temperature decreases, the
hole/electron-transfer rates for the monocations of the three types of dyads converge and then cross over. At the lowest
temperatures examined (160−170 K), the trend in the hole/electron-transfer rates is essentially reversed [k(Tl2-B) > k(Tl2-M) ∼
k(Tl2-U)]. The trends in the temperature dependence of hole/electron-transfer among the three dyads are consistent with
torsional motions of the aryl rings of the linker providing for thermal activation of the process at higher temperatures in the case
of the less torsionally constrained dyads, Tl2-U and Tl2-M. In the case of the most torsionally constrained dyad, Tl2-B, the hole/
electron-transfer process is activationless at all temperatures studied. The reversal in rates of hole/electron transfer among the
three dyads at low temperature is qualitatively explained by the results of density functional theory calculations, which predict
that static electronic factors could dominate the hole/electron-transfer process when torsional dynamics are thermally
diminished.
interactions observed in the EPR spectrum of the π-cation
radical. In early work, the hyperfine interactions due to the
naturally abundant 1H and 14N nuclei were used to “clock” the
hole/electron-transfer process.13,17−19 Subsequently, arrays
prepared with site-specific 13C labels were employed.20−22
More recently, we have found that 203Tl/205Tl hyperfine
I. INTRODUCTION
Understanding electronic communication among interacting
chromophores is essential for the rational design of molecular
architectures for artificial photosynthetic light harvesting and
energy conversion. Porphyrinic macrocycles have been widely
employed in the construction of synthetic light-harvesting
arrays owing to their attractive and versatile physical properties
and amenability to synthetic control.1−16 An effective light-
harvesting array absorbs intensely and transfers the resulting
electronic excited-state energy to a specific site with high
efficiency. Conversion of the harvested excited-state energy to
electrical energy then requires efficient electron injection into
the anode followed by efficient ground-state hole migration
away from the anode, thereby preventing charge recombina-
tion. Accordingly, understanding hole mobility in prototypical
light-harvesting and charge-separation systems is of fundamen-
tal interest.
1
“clocks” are greatly superior to those provided by H, 14N, or
13C owing to the fact that the 203Tl/205Tl hyperfine couplings
1
are much larger (15−25 G) than those of the H, 14N, or 13C
nuclei (1−6 G).23 The large 203Tl/205Tl hyperfine interactions
permit accurate simulations of the EPR spectra of the π-cation
radicals of the arrays and the extraction of specific rate
constants of hole/electron transfer.
We have applied the 203Tl/205Tl hyperfine-clock strategy to a
variety of porphyrin dyads that are joined at a meso position of
the porphyrin macrocycle via linkers of a range of lengths and
composition (diphenylethyne, diphenylbutadiyne, and (p-
Our groups have previously investigated ground-state hole/
electron transfer in oxidized porphyrinic arrays using EPR
spectroscopy.13 This approach entails analysis of the hyperfine
Received: July 23, 2012
© XXXX American Chemical Society
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Chart 1. Thallium(III) Porphyrin Dyads and Corresponding Monomers
phenylene)n where n = 1−4).23 The hole/electron-transfer time
constants at room temperature were found to be in the regime
of hundreds of picoseconds to subten nanoseconds, depending
on the specific porphyrin and/or linker. Variable-temperature
EPR studies were used to probe the activation energies of the
hole/electron-transfer process. These studies demonstrated
that the hole/electron-transfer process is typically weakly
activated (12−15 kJ mol−1) from room temperature down to
∼220 K; at temperatures in the 160−220 K range, the process
is essentially activationless. However, the detailed character-
istics of the activation behavior depend on the nature of the
linker. The (p-phenylene)n-linked dyads (n = 1−4) are
illustrative of this point. The dyad with a single p-phenylene
linker exhibits weakly activated hole/electron transfer over the
160−295 K temperature range. As the number of p-phenylenes
in the linker increases, the activation energy monotonically
decreases such that the dyad with four p-phenylenes in the
linker exhibits activationless hole/electron transfer over the full
temperature range studied. We have proposed that the
activation process for hole/electron transfer in the p-phenylene-
(and diphenylethyne- and diphenylbutadiyne-) linked dyads is
primarily due to restricted torsional motions of the aryl rings of
the linker.
to be tested in a systematic fashion. The p-phenylene-linked
dyads do not provide for such a test because the length of the
linker and the energy of the highest-occupied molecular orbital
(HOMO) of the linker also vary as the number of p-phenylene
units increases.23 These factors, as well as torsional constraints,
will contribute to the overall barrier for hole/electron transfer.
To minimize the contribution of linker length and HOMO
energetics to the hole/electron-transfer process, we report
herein the results of studies on a set of thallium porphyrin
dyads wherein the only significant difference in the linker is the
degree of torsional constraint. These dyads, designated Tl2-U,
Tl2-M, and Tl2-B, contain a diarylethyne linker wherein ortho-
dimethyl substituents are systematically added to the aryl rings
of the linker (Chart 1). Two representative monomeric
thallium porphyrins, Tl-U and Tl-H, were also examined
(Chart 1).
The rationale for the incorporation of ortho-methyl groups
on the meso-aryl rings is shown in Figure 1, which displays the
conformation of the meso-aryl ring relative to the porphyrin
macrocycle. The meso-aryl ring lacking any ortho-methyl
groups can rotate toward coplanarity with the porphyrin
macrocycle, thereby increasing π-conjugation and enhancing
the interporphyrin electronic communication that underpins
hole/electron transfer.24 In contrast, the meso-aryl ring that
bears two ortho-methyl groups is torsionally constrained toward
such rotation, whereupon the electronic communication is
The proposal that the activation process for hole/electron
transfer in the porphyrin dyads is due to restricted torsional
motions of the aryl rings in the linker, while reasonable, has yet
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Scheme 1. Rational Synthesis of Free Base Porphyrin
Building Blocks
Figure 1. Absence or presence of torsional constraints on meso-aryl
rotation.
expected to diminish. Accordingly, studies of the dyads shown
in Chart 1 unambiguously elucidate the origin of the activation
process for hole/electron transfer in the porphyrin dyads.
II. RESULTS
A. Synthesis. A set of meso-substituted porphyrins was
prepared wherein each porphyrin bears phenyl groups at the
5,10,15-positions and an ethynylaryl or iodoaryl group at the
20-position. The ethynylaryl or iodoaryl groups are unsub-
stituted at the 2,6-positions or bear 2,6-dimethyl groups to
inhibit rotation of the aryl group toward coplanarity with the
porphyrins.24 Each of the porphyrins bearing an iodo group
(Fb-H-I,24 Fb-U-I25,26) or trimethylsilylethynyl group (Fb-H-
TMS,24 Fb-U-TMS27) has been prepared previously using
statistical techniques. Here, the same porphyrins were prepared
in a rational manner by coupling dipyrromethane-dicarbinol (1-
OH) and a dipyrromethane (2a−d) respectively, using a
literature procedure.28 The procedure entailed reaction in
CH2Cl2 containing Sc(OTf)3 (2.8 mM) and 2,6-di-tert-
butylpyridine (2,6-DTBP) (28 mM) at room temperature
followed by oxidation with DDQ. The putative dipyrro-
methane-dicarbinol (1-OH) was obtained by the reduction of
diacyldipyrromethane−tin complex 1 using NaBH4 (Scheme
1).28−30
Treatment of the trimethylsilylethynylporphyrins Fb-H-TMS
and Fb-U-TMS to fluoride-mediated deprotection (using
tetrabutylammonium fluoride) in THF unveiled the ethyne
moiety and afforded porphyrins Fb-H and Fb-U, respectively.
The latter porphyrin also has been prepared previously,27,31
whereas the former has been employed as the zinc chelate.24
The thallium(III) complexes of porphyrins Fb-H and Fb-U
were prepared by treating the free base with excess TlCl3·4H2O
(Scheme 2).
and P(o-tol)3 in toluene/triethylamine (5:1) at 40 °C for 6 h
afforded Fb2-B, Fb2-M, and Fb2-U in 47%, 41%, and 28% yield,
respectively. Treatment of the three free base dyads with excess
TlCl3·4H2O in CH2Cl2/CH3OH at room temperature34 for 6 h
provided the corresponding thallium dyads in 51−62% yield
(Scheme 3).
B. EPR Spectra of the Monocations of the Thallium-
Chelated Monomers. The room-temperature EPR spectra of
the monocations of Tl-U and Tl-H are shown in Figure 2. The
EPR spectra of the thallium-chelated porphyrins exhibit a
simple hyperfine doublet arising from coupling of the unpaired
electron spin with the I = 1/2 thallium nuclei. The 203Tl/205Tl
hyperfine coupling for the monocations of Tl-U and Tl-H are
11.1 and 16.8 G, respectively. No additional hyperfine structure
is observed; such structure might have arisen due to resolving
of the hyperfine doublets of the 203Tl and 205Tl nuclei or
resolving of the nine-line pattern expected from the 14N nuclei.
These characteristics of the EPR spectra of the monocations of
The meso-substituted diphenylethyne-linked porphyrin dyads
were obtained by the copper-free, palladium mediated
Sonogashira coupling of an iodophenylporphyrin and an
ethynylphenylporphyrin.32,33 The reaction of Fb-H and Fb-
H-I, Fb-U and Fb-H-I, and Fb-U and Fb-U-I with Pd2(dba)3
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the thallium-chelated monomers are similar to those we have
previously observed for the thallium chelates of other types of
porphyrins.23 [Note that the 14N hyperfine couplings reported
in Figure 2 and elsewhere herein are obtained from simulations
that indicate that inclusion of this coupling gives a better fit to
the line shape.]
Scheme 2. Synthesis of Thallium(III)-Containing Porphyrin
Building Blocks
The observation that the presence of an ortho-dimethylaryl
group affects the magnitude of 203Tl/205Tl hyperfine couplings
(Tl−U, 11.1G; Tl−H, 16.8 G) is further consistent with our
previous studies on thallium-chelated porphyrins that contain
three mesityl groups at the non-linking meso positions of the
porphyrin ring and an aryl ring that lacks any ortho-methyl
substituents at the linker position.23 The 203Tl/205Tl hyperfine
couplings for the monocation of this latter porphyrin (24.9 G),
which contains three meso groups with ortho-dimethylaryl
substitution, are larger than those for either Tl−U (11.1 G) or
Tl−H (16.8 G), which contain zero or one group with ortho-
dimethylaryl substitution, respectively. The 203Tl/205Tl hyper-
fine couplings in the monocation of thallium-chelated
tetramesitylporphyrin, which contains four meso groups with
ortho-dimethylaryl substitution, are larger still (27 G) and
further consistent with the trend (unpublished results).
One possible explanation for the increase in 203Tl/205Tl
hyperfine couplings as the number of meso ortho-dimethylaryl
substituents increases is that steric interactions between the
methyl groups and the thallium ion force the metal more in-
plane thereby enhancing the degree of metal−porphyrin orbital
mixing. However, this view is not consistent with the results of
density functional theory (DFT) calculations on the various
porphyrins that predict that the extent of doming of the
porphyrin is not affected by the addition of ortho-dimethylaryl
substituenst. Accordingly, the change in the magnitude of the
203Tl/205Tl hyperfine couplings as the number of meso ortho-
dimethylaryl substituents increases must arise from subtle
electronic effects. These electronic effects must indeed be small
because the potentials of the first oxidation of Tl-U and Tl-H
Scheme 3. Directed Synthesis of Diarylethyne-Linked
Thallium(III) Porphyrin Dyads
are the same to within experimental error (<
0.02 V).
Likewise the energies of the HOMOs of the two porphyrins are
essentially identical (vide infra).
C. EPR Spectra of the Monocations of the Tl2-U, Tl2-M,
and Tl2-B Dyads. The variable-temperature EPR spectra of
the monocations of Tl2-U, Tl2-M, and Tl2-B are shown in the
left panels of Figures 3, 4, and 5, respectively. For all three
dyads, spectra are not shown at temperatures below the 160−
170 K range because the solvent/electrolyte mixture freezes
and the spectra change abruptly and become extremely broad.
Spectra were not obtained above the 320−340 K range owing
to solvent volatility.
The EPR spectra of the monocations of Tl2-U (Figure 3) and
Tl2-M (Figure 4) are qualitatively similar to one another. The
spectra both exhibit a three-line 203Tl/205Tl hyperfine pattern at
room temperature, which gradually evolves into a two-line
hyperfine pattern as the temperature is lowered. The EPR
spectra of the monocation of Tl2-B (Figure 5) are somewhat
different in that the three-line pattern 203Tl/205Tl hyperfine
pattern is much less pronounced at higher temperatures.
Simulations of the variable-temperature EPR spectra of the
monocations of Tl2-U, Tl2-M, and Tl2-B are shown on the right
sides of Figures 3, 4, and 5, respectively. The 203Tl/205Tl and
14N hyperfine couplings and line widths used for all the
simulations are compiled in Table S1 (Supporting Informa-
tion). The hole/electron-transfer rate constants derived from
the simulations are shown above each spectral trace. At room
Figure 2. Room-temperature EPR spectra of the monocations of Tl-U
and Tl-H.
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Figure 5. Variable-temperature EPR spectra of the monocation of Tl2-
B (left) and simulated spectra (right) with derived hole-transfer rates.
Figure 3. Variable-temperature EPR spectra of the monocation of Tl2-
U (left) and simulated spectra (right) with derived hole-transfer rates.
Figure 6. Marcus-equation plots for the monocations of Tl2-U, Tl2-M,
and Tl2-B.
where ΔG‡ = (ΔGo + λ)2/4λ; A = (4π3/h2λkB)1/2|V2|; ΔGo is
the free energy change associated with hole/electron transfer; λ
is the reorganization energy; and V is the effective electronic
coupling. Both Tl2-U and Tl2-B are symmetrical; thus, the
electron-donor and -acceptor thallium porphyrins are redox
equivalent (isoenergetic) and the hole/electron-transfer process
has ΔGo = 0. Tl2-M is not rigorously symmetrical; however, as
noted above, the redox potentials of the constituent porphyrins
are the same to within experimental error. Likewise, the
difference in the energies of HOMOs of the constituent
porphyrins is minimal (vide infra). Thus, the hole/electron
transfer in the monocation of this dyad is also taken to have
ΔGo = 0.
Inspection of Figure 6 reveals the following key features.
(1) The plot for the monocation of Tl2-U exhibits a positive
slope over the 170−320 K temperature range. The data are well
fit with eq 1; however, nearly equivalent fits are obtained using
eq 1 in the adiabatic limit (no T dependence in the prefactor).
The values of ΔG‡ ∼ 14.6 kJ/mol and A ∼ 3.1 × 1012 K s−1 are
Figure 4. Variable-temperature EPR spectra of the monocation of Tl2-
M (left) and simulated spectra (right) with derived hole-transfer rates.
temperature (295 K), the hole/electron-transfer rate constants
for Tl2-U, Tl2-M, and Tl2-B are 5.1 × 108 s−1, 3.3 × 108 s−1, and
1.6 × 108 s−1, respectively. The simulations further indicate that
the decrease in rate as the temperature is lowered becomes less
pronounced as ortho-dimethyl groups are added to the aryl
group of the linker. Indeed, for the monocation of Tl2-B, the
hole/electron-transfer rate does not change appreciably over
the 160−340 K temperature range.
To probe further the temperature dependence of hole/
electron-transfer, the rate constants were plotted logarithmi-
cally versus 1/T. Plots of kT1/2 versus 1/T for the monocations
of Tl2-U, Tl2-M, and Tl2-B are shown in Figure 6. This
approach assumes hole/electron transfer can be modeled using
the semiclassical Marcus equation in the nonadiabatic limit,35
k = AT−1/2exp(−ΔG‡/kBT)
(1)
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obtained from the slope and intercept of the linear fits to the
data using eq 1.
the two porphyrins are essentially identical (−5.25 eV). This
result supports the approximation that ΔGo ∼ 0 for the hole/
electron-transfer process in the monocation of Tl2-M. The
calculations for the U, M, and B linkers further predict that the
HOMO energies change relatively modestly as ortho-dimethyl
groups are added to the basic diarylethyne structure (−5.46 eV
versus −5.36 eV versus −5.27 eV).
(2) The plot for the monocation of Tl2-M differs somewhat
from that of Tl2-U in that only the region above 240 K is well
fit via eq 1 (or the adiabatic equivalent) and yields ΔG‡ ∼ 16.8
kJ mol−1 and A ∼ 5.1 × 1012 K s−1. Below 240 K, the data are
somewhat irregular, exhibiting a generally less positive slope
than the data above 240 K.
(3) The plot for the monocation of Tl2-B is essentially flat
over the 160−340 K temperature range, albeit there appears to
be an onset of a slight positive slope above 300 K. Regardless,
the data are indicative of an essentially activationless hole/
electron-transfer process (ΔG‡ < kBT).
III. DISCUSSION
We have examined the hole/electron-transfer process in a series
of singly oxidized thallium-chelated porphyrin dyads wherein
the torsional constraints in the linker are systematically
increased (Tl2-U < Tl2-M < Tl2-B). Many of the general
characteristics of the hole/electron-transfer process in the
monocations of these dyads are similar to those we have
previously reported for other types of thallium-chelated
porphyrin dyads.23 We will not focus on these characteristics
herein. Instead, the reader is referred to our previous work for
detailed discussion.
The key observations from the present studies are as follows:
(1) Hole/electron-transfer at room temperature (295 K) in the
monocations of Tl2-U, Tl2-M, and Tl2-B occurs at rates that
correspond to time scales in the 2−5 ns regime. (2) Hole/
electron transfer at room temperature (295 K) slows as
torsional constraints are added to the basic diarylethyne linker
[k(Tl2-U) > k(Tl2-M) > k(Tl2-B)]. (3) As the temperature
decreases, the hole/electron-transfer rates for the monocations
of the three types of dyads converge and then cross over. At the
lowest temperatures examined (160−170 K), the trend in the
hole/electron-transfer rates is essentially reversed [k(Tl2-B) >
k(Tl2-M) ∼ k(Tl2-U)].
D. Molecular Orbital Characteristics of the Thallium
Porphyrins and Linkers. To gain insight into the relative
energies of the molecular orbitals of the porphyrins and linkers
in the dyads, DFT calculations were performed on a series of
model compounds. The focus of these calculations was the
relative energies of the HOMOs of the molecules, which are the
key orbitals for the hole/electron-transfer process. The two
porphyrin model compounds included a thallium chelate with
four meso-phenyl substituents and a thallium chelate with three
meso-phenyl and one meso-mesityl substituents. The three
linker model compounds included 1,2-di-p-tolylethyne (U),
1,2-bis(3,4,5-trimethylphenyl)ethyne (B), and an ethynyl
species containing one p-tolyl and one 3,4,5-trimethylphenyl
group (M). The terminal methyl group (para position in tolyl,
4-position in 3,4,5-trimethylphenyl) is included to represent the
20-carbon of the porphyrin to which the linker would be
attached; thus, the p-tolyl group models the unsubstituted aryl
unit whereas the 3,4,5-trimethylphenyl group models the aryl
unit bearing two o-methyl groups.
The trends in the hole/electron-transfer rates exhibited by
the monocations of Tl2-U, Tl2-M, and Tl2-B as a function of
temperature arise because the thermal-activation behavior of
the process is different among the three types of dyads. In the
case of the monocation of Tl2-U, hole/electron transfer is
(weakly) thermally activated over the full temperature range
examined, whereas for the monocation of Tl2-B, hole/electron
transfer is essentially activationless over this temperature range.
In the case of the monocation of Tl2-M, hole/electron transfer
is (weakly) activated at room temperature (and somewhat
below), but appears to trend toward a less-activated
(activationless) regime at lower temperature. In this respect,
the thermal-activation behavior exhibited by the monocation of
Tl2-M is intermediate between that exhibited by the
monocations of Tl2-U and Tl2-B.
The calculated electron densities and energies for the
HOMOs and lowest unoccupied molecular orbitals
(LUMOs) of the two porphyrins and three linkers are shown
in Figure 7. The structures of the molecules are included in the
figure. The calculations predict that the HOMO energies for
We have previously proposed that the most self-consistent
explanation for the activation behavior observed for the hole/
electron-transfer process in the monocations of the porphyrin
dyads is that torsional motions of the aryl rings in the linker
contribute to the process. In this picture, rotation of the aryl
rings toward coplanarity with the porphyrin would enhance
hole/electron transfer via an increase in a conjugative pathway
between the two porphyrins. The results of the present study
are consistent with this view. The addition of ortho-dimethyl
groups to the basic diarylethyne linker, which substantially
increases the torsional barrier for ring rotation,36 appears to
systematically diminish the thermal activation of hole/electron-
transfer. Indeed, in the case of the monocation of Tl2-B, in
which both aryl rings of the linker are highly constrained from
tending toward coplanarity with the porphyrin ring, the hole/
electron-transfer process is essentially activationless.
Figure 7. Calculated electron densities and energies for the HOMOs
and LUMOs of the U, M, and B linkers and of two model thallium
porphyrins.
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As we have previously discussed,23 the ΔG‡ values
determined for all the porphyrin dyads are much smaller than
the measured aryl-ring torsional barriers in meso-substituted
porphyrins. These barriers are in the 60−100 kJ mol−1 range.36
Thus, any type of torsional motion that might contribute to
activation of the hole/electron-transfer process in the
monocations of the porphyrin dyads would necessarily involve
only restricted rotation of the aryl ring toward coplanarity with
the porphyrin. In particular, the faster rates at higher
temperature are due to an increase in the amplitude of the
low-frequency, large-amplitude torsional motions that occurs
upon population of higher vibrational states in the torsional
potential energy well.
One characteristic of the thermal behavior of the hole/
electron-transfer process in the monocations of Tl2-U, Tl2-M,
and Tl2-B that is puzzling is the reversal in the rates along the
series of dyads as the temperature is lowered. One plausible
explanation is that electronic rather than torsional effects
dominate at lower temperatures. This view is qualitatively
consistent with the relative energies of the HOMOs of the U,
M, and B linkers relative to those of the thallium-chelated
porphyrins (Figure 7). As ortho-dimethyl groups are added to
the linker, the energy of the linker HOMO becomes less
negative and approaches (but remains lower than) that of the
thallium-chelated porphyrins. The most consistent explanation
is if the linker HOMOs remain sufficiently below those of the
porphyrins so that hole/electron transfer proceeds by a
superexchange mechanism (tunneling rather than hopping).
Within that mechanism, as we have previously discussed, the
difference in energies (ΔEPL) of the HOMOs of the linker and
porphyrin are related to the height of the tunneling barrier for
hole/electron transferthe smaller the ΔEPL the lower the
tunneling barrier.23 The prediction that the value of ΔEPL
becomes smaller along the series [ΔEPL(Tl2-U) > ΔEPL(Tl2-M)
> ΔEPL(Tl2-B)] would be consistent with the monocation of
Tl2-B exhibiting the fastest rate at lower temperature. This
picture is basically a static view of the hole/electron-transfer
process. As the temperature increases, torsional dynamics come
into play and then dominate the hole/electron-transfer process
at more elevated temperatures.
transduction processes and to information storage in molecular
electronics. In the architectures studied herein, the diarylethyne
linker provides a conduit for ground-state hole/electron
transfer between two porphyrins. Pinpointing the rates of
hole/electron transfer has heretofore proved somewhat
difficult. Application of the recently demonstrated 203Tl/205Tl
hyperfine “clocking” strategy to a series of three diarylethyne-
linked porphyrin dyads enabled determination of the rates of
ground-state hole/electron transfer, and, in conjunction with
variable temperature studies, delineated the effects of torsional
constraints on such rates.
The finding that ground-state hole/electron transfer at room
temperature (295 K) slows as torsional constraints are added to
the diarylethyne linker [k(Tl2-U) > k(Tl2-M) > k(Tl2-B)] is
not surprising, but knowledge of the exact rate constants (k
ranges from 5.1 × 108 s−1 to 1.6 × 108 s−1 at 295 K) was
heretofore not known. The change in rate constant by a factor
of ∼3.2 is consistent with results for excited-state energy
transfer, excited-state hole−electron transfer, and ground-state
hole/electron transfer in the same molecular architectures
wherein one porphyrin is a zinc chelate and the other is a free
base.13,18,37a,b The excited-state energy transfer process also is
dominated by a through-bond mechanism mediated by the
diarylethyne linker. The general time scale and diminution in
rate constant found here for ground-state hole/electron transfer
between thallium porphyrins are consistent with the values
obtained at 295 K using transient absorption spectroscopy for
zinc porphyrins with no steric constraints (k = 12.5 × 108 s−1)
or steric hindrance on one aryl ring of the linker (k = 8.3 × 108
s−1).37c Variable-temperature studies here have revealed the
subtle interplay of steric factors (torsional constraints on the
linker) and electronic factors for ground-state hole/electron
transfer; thus, the energy of activation for hole/electron transfer
(ΔG‡) is ∼14.6 kJ/mol for Tl2-U, ∼ 16.8 kJ mol−1 for Tl2-M,
and < kBT (i.e., essentially activationless) for Tl2-B. Such data
should be of value in the rational design of multicomponent
molecular architectures that convey holes/electrons in diverse
schemes for artificial photosynthesis and molecular electronics.
V. EXPERIMENTAL SECTION
Finally, we note that the temperature dependence observed
in our previous study of hole/electron transfer in the
monocations of a series of (p-phenylene)n-linked dyads parallels
that observed for the monocations of Tl2-U, Tl2-M, and Tl2-B.
The rates for the (p-phenylene)n-linked dyads at room
temperature monotonically decrease as the number of p-
pheneylene units in the linker increases from one to four. As
the temperature is lowered, the rates cross over, and at low
temperature, the trend is reversed. However, the calculated
ΔEPL values for the p-phenylene linkers also monotonically
decrease as the number of p-phenylene units in the linker
increases. Consequently, the barrier to hole/electron-transfer
would become lower. Collectively, this interplay of electronic
and torsional effects might then also explain the temperature
dependence of the activation behavior of hole/electron transfer
in the monocations of the (p-phenylene)n-linked dyads.
A. General Methods. 1H NMR spectra (300 MHz) were collected
at room temperature in CDCl3 unless noted otherwise. Silica gel (40
μm average particle size) was used for column chromatography. Matrix
assisted laser desorption ionization mass spectrometry (MALDI-MS)
employed a matrix of 1,4-bis(5-phenyl-2-oxazol-2-yl)benzene.38
Electrospray ionization mass spectrometry (ESI-MS) data are reported
for the molecular ion or protonated molecular ion. The palladium-
coupling reactions were carried out using Schlenk-line techniques.
Size-exclusion chromatography (SEC) was performed as described
previously.39
B. Solvents. All solvents were reagent grade and were used as
received unless noted otherwise. THF was freshly distilled from
sodium/benzophenone ketyl.
C. Noncommercial Compounds. Dipyrromethanes 1,29 2a,40,41
2b,41 2c,40,41 and 2d42 were prepared as described in the literature.
D. Electrochemistry. The electrochemical measurements were
performed using techniques and instrumentation previously de-
scribed.43 The sample concentrations were at levels that preclude
intermolecular disproportionation reactions which would result in
formation of dication radical species.17,18 The samples were contained
in a three-compartment cell equipped with a Pt wire working
electrode, a Pt mesh counter electrode, and a Ag/Ag+ (butyronitrile)
reference electrode. The solvent was CH2Cl2 containing 0.1 M
Bu4NPF6 as the supporting electrolyte. All reported potentials are with
respect to ferrocene/ferrocenium = 0.19 V. The bulk-oxidized
IV. OUTLOOK
The rational design of molecular architectures that support
artificial photosynthesis and molecular electronics requires a
deep understanding of the factors that affect electronic
interactions between constituent chromophores. Ground-state
hole/electron transfer is central to many solar energy-
G
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complexes were prepared in a glovebox via quantitative coulometric
bulk electrolysis at a Pt mesh electrode. The integrity of the samples
was checked by cyclic voltammetry after each successive oxidation.
Upon oxidation, the samples were transferred to an EPR tube and
sealed in the glovebox.
E. EPR Spectroscopy and Simulations. The EPR spectra were
recorded on an X-band spectrometer (Bruker EMX) equipped with an
NMR gaussmeter and microwave frequency counter. The EPR spectra
were obtained on samples that were typically 0.2 mM. The microwave
power and magnetic field modulation amplitude were typically 5.7 mW
and 0.32 G, respectively.
5 min and then filtered through filter paper. The filtered material was
washed with methanol and dried in vacuo to afford a crystalline purple
solid (245 mg, 16%): 1H NMR (300 MHz, CDCl3) δ −2.78 (br, 2H),
0.37 (s, 9H), 7.68−7.80 (m, 9H), 7.86 (d, J = 8.1 Hz, 2H), 8.16 (d, J =
8.1 Hz, 2H), 8.18−8.24 (m, 6H), 8.78−8.88 (m, 8H); MALDI-MS
obsd 711.0; ESI-MS obsd 711.2938, calcd 711.2939 [(M + H)+, M =
C49H38N4Si]; λabs (CH2Cl2) 418, 515, 551, 589 nm. The data are in
close agreement with those for the preparation via an alternative
route.27
5,10,15-Triphenyl-20-[2,6-dimethyl-4-ethynylphenyl]porphyrin
(Fb-H). A solution of porphyrin Fb-H-TMS (147 mg, 200 μmol) in
THF (20 mL) was treated with 1 M TBAF in THF (300 μL, 300
μmol). The reaction mixture was stirred at room temperature for 1 h
under argon. The reaction mixture was concentrated to dryness. The
resulting purple solid was dissolved in CH2Cl2 (50 mL). The organic
solution was washed with saturated aqueous NaHCO3 and water, dried
(Na2SO4), and concentrated. Column chromatography [silica,
The EPR spectra of the monocations of the monomers were
simulated with the WinSim program to obtain the values of the
203/205Tl and 14N hyperfine coupling constants and linewidths.44 The
EPR spectra of the monocations of the dyads were simulated with the
ESR-EXN program.45,46 This program and its application to extract
exchange rates from isotropic EPR spectra have been described in
detail elsewhere.45 The hyperfine coupling constants and line widths
obtained from the simulations of the spectra of the monocations of the
monomers were used as the initial values of these parameters in the
simulations of the spectra of the monocations of the dyads. Prior to
initiating the latter simulations, it was confirmed that the EPR spectra
of the monocations of the monomers were faithfully reproduced by the
ESR-EXN program using the parameters obtained from the WinSim
program. The hole/electron-transfer rates at the different temperatures
were derived from the simulations of the variable-temperature EPR
spectra of the monocations of the dyads. The rate constants were
adjusted manually to provide a best fit to the spectra as judged by the
residuals between the observed and simulated spectra. Changes in the
rate constants of 10% from the best fit values resulted in clearly
poorer fits to the spectra. Finally, the hyperfine coupling constants and
line widths vary somewhat with temperature (a few percent);
consequently, these parameters were adjusted to optimize the fits of
the EPR spectra obtained at the different temperatures.
1
CH2Cl2/hexanes (1:1)] provided a purple solid (112 mg, 84%): H
NMR (300 MHz, CDCl3) δ −2.70 (br, 2H), 1.86 (s, 6H), 3.24 (s,
1H), 7.63 (s, 2H), 7.68−7.82 (m, 9H), 8.18−8.26 (m, 6H), 8.65 (d, J
= 4.8 Hz, 2H), 8.82 (d, J = 4.8 Hz, 2H), 8.83 (s, 4H); MALDI-MS
obsd 666.5; ESI-MS obsd 667.2847, calcd 667.2856 [(M + H)+, M =
C48H34N4]; λabs (CH2Cl2) 418, 485, 513, 548, 592 nm.
5,10,15-Triphenyl-20-(4-ethynylphenyl)porphyrin (Fb-U). A sol-
ution of porphyrin Fb-U-TMS (142 mg, 200 μmol) in THF (20 mL)
was treated with 1 M TBAF in THF (300 μL, 300 μmol). The reaction
mixture was stirred at room temperature for 1 h under argon. The
reaction mixture was concentrated to dryness. The resulting purple
solid was dissolved in CH2Cl2 (50 mL). The organic solution was
washed with saturated aqueous NaHCO3 and water, dried (Na2SO4)
and concentrated. Column chromatography [silica, CH2Cl2/hexanes
1
(1:1)] provided a purple solid (112 mg, 88%): H NMR (300 MHz,
CDCl3) δ −2.79 (br, 2H), 3.31 (s, 1H), 7.70−7.80 (m, 9H), 7.89 (d, J
= 8.4 Hz, 2H), 8.16−8.24 (m, 8H), 8.78−8.88 (m, 8H); MALDI-MS
obsd 638.3; ESI-MS obsd 639.2541, calcd 639.2543 [(M + H)+, M =
C46H30N4]; λabs (CH2Cl2) 418, 515, 550, 591 nm. The data are in close
agreement with those for the preparation via an alternative route.27,31
5,10,15-Triphenyl-20-(2,6-dimethyl-4-iodophenyl)porphyrin (Fb-
H-I). Following a reported procedure,28 the condensation of dicarbinol
1-OH [derived from reduction of 1 (952 mg, 1.44 mmol) with NaBH4
(2.72 g, 72.0 mmol) in THF/MeOH (115 mL, 3:1] and dipyrro-
methane 2c (541 mg, 1.44 mmol) was carried out in the presence of
2,6-di-tert-butylpyridine (420 μL, 1.87 mmol) and Sc(OTf)3 (92.0 mg,
0.187 mmol) in CH2Cl2 (67 mL) at room temperature for 10 min
followed by the addition of DDQ (980 mg, 4.32 mmol). After 5 min,
the reaction mixture was filtered through a pad of alumina (CH2Cl2
containing 0.1% triethylamine). The resulting porphyrin-containing
solution was concentrated to give a purple solid. Methanol was added.
The resulting suspension was sonicated for 5 min and then filtered
through filter paper. The filtered material was washed with methanol
and dried in vacuo to afford a crystalline purple solid (188 mg, 17%):
1H NMR, mass, and UV−vis spectral data were consistent with our
F. Molecular Orbital Calculations. DFT calculations were
performed with Spartan '08 for Windows 1.2.047 in parallel mode on
a PC equipped with an Intel i7-975 CPU, 24 GB ram, and three 300-
GB, 10k-rpm hard drives. The hybrid B3LYP functional and the
LACVP basis set were employed. The equilibrium geometries were
fully optimized using the default parameters of the Spartan '08
program.
G. Synthesis Procedures. 5,10,15-Triphenyl-20-[2,6-dimethyl-4-
[2-(trimethylsilyl)ethynyl]phenyl]porphyrin (Fb-H-TMS). Following a
reported procedure,28 the condensation of dicarbinol 1-OH [derived
from reduction of 1 (1.09 g, 1.65 mmol) with NaBH4 (1.24 g, 33.0
mmol) in THF/MeOH (132 mL, 3:1)] and dipyrromethane 2a (571
mg, 1.65 mmol) was carried out in the presence of 2,6-di-tert-
butylpyridine (481 μL, 2.14 mmol) and Sc(OTf)3 (105 mg, 0.214
mmol) in CH2Cl2 (76 mL) at room temperature for 10 min followed
by the addition of DDQ (1.12 g, 4.95 mmol). After 5 min, the reaction
mixture was filtered through a pad of alumina (CH2Cl2 containing
0.1% triethylamine). The resulting porphyrin-containing solution was
concentrated to give a purple solid. Methanol was added. The resulting
suspension was sonicated for 5 min and then filtered through filter
paper. The filtered material was washed with methanol and dried in
data for the preparation via an alternative route.24
5,10,15-Triphenyl-20-(4-iodophenyl)porphyrin (Fb-U-I). Follow-
ing a reported procedure,28 the condensation of dicarbinol 1-OH
[derived from reduction of 1 (569 mg, 860 μmol) with NaBH4 (1.62 g,
43.0 mmol) in THF/MeOH (69 mL, 3:1)] and dipyrromethane 2d
(300 mg, 860 μmol) was carried out in the presence of 2,6-di-tert-
butylpyridine (251 μL, 1.12 mmol) and Sc(OTf)3 (55.0 mg, 0.112
mmol) in CH2Cl2 (40 mL) at room temperature for 10 min followed
by the addition of DDQ (586 mg, 2.58 mmol). After 5 min, the
reaction mixture was filtered through a pad of alumina (CH2Cl2
containing 0.1% triethylamine). The resulting porphyrin-containing
solution was concentrated to give a purple solid. Methanol was added.
The resulting suspension was sonicated for 5 min and then filtered
through filter paper. The filtered material was washed with methanol
and dried in vacuo to afford a crystalline purple solid (92 mg, 14%):
1H NMR (300 MHz, CDCl3) δ −2.80 (br, 2H), 7.68−7.82 (m, 9H),
7.94 (d, J = 8.1 Hz, 2H), 8.08 (d, J = 8.1 Hz, 2H), 8.18−8.26 (m, 6H),
8.78−8.90 (m, 8H); MALDI-MS obsd 741.1; ESI-MS obsd 741.1503,
1
vacuo to afford a crystalline purple solid (231 mg, 19%): H NMR,
mass, and UV−vis spectral data were consistent with our data for the
preparation via an alternative route.24
5,10,15-Triphenyl-20-[4-[2-(trimethylsilyl)ethynyl]phenyl]-
porphyrin (Fb-U-TMS). Following a reported procedure,28 the
condensation of dicarbinol 1-OH [derived from reduction of 1 (1.43
g, 2.16 mmol) with NaBH4 (4.08 g, 108 mmol) in THF/MeOH (175
mL, 3:1)] and dipyrromethane 2b (688 mg, 2.16 mmol) was carried
out in the presence of 2,6-di-tert-butylpyridine (628 μL, 2.80 mmol)
and Sc(OTf)3 (138 mg, 0.280 mmol) in CH2Cl2 (100 mL) at room
temperature for 10 min followed by the addition of DDQ (1.47 g, 6.48
mmol). After 5 min, the reaction mixture was filtered through a pad of
alumina (CH2Cl2 containing 0.1% triethylamine). The resulting
porphyrin-containing solution was concentrated to give a purple
solid. Methanol was added. The resulting suspension was sonicated for
H
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calcd 741.1510 [(M + H)+, M = C44H29IN4]; λabs (CH2Cl2) 418, 514,
549, 590 nm. The data are in close agreement with those for the
preparation via an alternative route.25,26
concentrated to dryness under reduced pressure without any heat. The
resulting crude product was chromatographed [silica, CH2Cl2/hexanes
(1:1)] to give a green solid (16.4 mg, 55%): H NMR (300 MHz,
1
1,2-Bis[4-(5,10,15-triphenylporphin-20-yl)-3,5-dimethylphenyl]-
ethyne (Fb2-B). Following a copper-free procedure for Sonogashira
coupling of porphyrins,32,33 samples of Fb-H (35.5 mg, 53.2 μmol),
Fb-H-I (40.8 mg, 53.2 μmol), and P(o-tol)3 (19.4 mg, 63.8 μmol) in
toluene/triethylamine [(5:1), 21 mL) were degassed by three freeze/
pump/thaw cycles. Then Pd2(dba)3 (7.3 mg, 8.0 μmol) was added,
and the mixture was additionally degassed by three freeze/pump/thaw
cycles. The resulting mixture was stirred for 6 h at 40 °C in a Schlenk
line. The reaction mixture was allowed to cool and then was
concentrated. The resulting crude product was chromatographed
(silica, CH2Cl2) to afford unreacted porphyrin monomers followed by
the desired dyad and then high molecular weight material (HMWM).
The mixture of dyad and HMWM was concentrated to dryness,
dissolved in THF, and chromatographed (SEC, THF) with gravity
elution. The dyad-containing fractions were combined and chromato-
graphed [silica, CH2Cl2/hexanes (1:1)] to afford a purple solid (33
CDCl3) δ 3.33 (s, 1H), 7.70−7.86 (m, 9H), 7.92 (dd, J(H−H) = 17.6
Hz and 7.8 Hz, 2H), 8.12 (d, J(H−H) = 6.6 Hz, 4H), 8.30−8.42 (m,
4H), 9.03 (d, J(Tl−H) = 64.8 Hz, 2H), 9.04 (d, J(Tl−H) = 64.5 Hz,
2H), 9.06 (d, J(Tl−H) = 64.8 Hz, 2H), 9.08 (d, J(Tl−H) = 64.2 Hz,
2H); MALDI-MS obsd 876.5; ESI-MS obsd 839.2032, calcd 839.2027
[({M − Cl}+ H)+, M = C46H27ClN4Tl]; λabs(CH2Cl2) 434, 566, 606
nm.
Dichloro[1,2-bis[4-(5,10,15-triphenylporphinatothallium(III)-20-
yl)-3,5-dimethylphenyl]ethyne] (Tl2-B). Following a reported proce-
dure,34 a solution of Fb2-B (15.7 mg, 12.0 μmol) in CH2Cl2 (10 mL)
was treated with TlCl3·4H2O (230 mg, 600 μmol) in CH3OH (1 mL)
at room temperature for 6 h. Then the reaction mixture was diluted
with CH2Cl2 (25 mL) and washed with saturated aqueous NaHCO3
and H2O. The organic layer was separated, dried (Na2SO4), filtered,
and concentrated to dryness under reduced pressure without any
applied heat. Subsequent chromatography [silica, CH2Cl2/hexanes
(1:1)] gave a green solid (12.4 mg, 58%): 1H NMR (300 MHz,
CDCl3) δ 1.89 (s, 6H), 2.03 (s, 6H), 7.68−7.88 (m, 22H), 8.13 (dd,
J(H−H) = 16.6 Hz and 7.5 Hz, 6H), 8.40 (d, J(H−H) = 3.3 Hz, 6H),
8.99 (dd, J(H−H), = 4.8 Hz, J(Tl−H) = 61.5 Hz, 4H), 9.06 (d, J(Tl−
H) = 65.7 Hz, 8H), 9.09 (dd, J(H−H), = 4.8 Hz, J(Tl−H) = 63.0 Hz,
4H); MALDI-MS obsd 1786.6, calcd exact mass 1782.4; λabs (CH2Cl2)
436, 566, 606 nm.
1
mg, 47%): H NMR (300 MHz, CDCl3) δ −2.65 (br, 4H), 1.96 (s,
12H), 7.68−7.81 (m, 18H), 7.82 (s, 4H), 8.20 (s, 4H), 8.25 (dd, J =
7.8 Hz and 1.8 Hz, 8H), 8.77 (d, J = 4.8 Hz, 4H), 8.86 (s, 8H), 8.88
(d, J = 4.8 Hz, 4H); MALDI-MS obsd 1307.8, calcd exact mass 1306.5;
λabs (CH2Cl2) 421, 512, 548 nm.
1-[4-(5,10,15-Triphenylporphin-20-yl)-3,5-dimethylphenyl]-2-[4-
(5,10,15-triphenylporphin-20-yl)phenyl]ethyne (Fb2-M). Following a
copper-free procedure for Sonogashira coupling of porphyrins,32,33
samples of Fb-U (31.9 mg, 50.0 μmol), Fb-H-I (38.4 mg, 50.0 μmol),
and P(o-tol)3 (18.2 mg, 60.0 μmol) in toluene/triethylamine [(5:1),
20 mL) were degassed by three freeze/pump/thaw cycles. Then
Pd2(dba)3 (6.8 mg, 7.5 μmol) was added, and the mixture was
additionally degassed by three freeze/pump/thaw cycles. The resulting
mixture was stirred for 6 h at 40 °C in a Schlenk line. The reaction
mixture was allowed to cool and then concentrated. The resulting
crude product was chromatographed (silica, CH2Cl2) to afford
unreacted porphyrin monomers followed by the desired dyad and
then HMWM. The mixture of dyad and HMWM was concentrated to
dryness, dissolved in THF, and chromatographed (SEC, THF) with
gravity elution. The dyad-containing fractions were combined and
chromatographed [silica, CH2Cl2/hexanes (1:1)] to afford a purple
Dichloro[1-[4-(5,10,15-triphenylporphinatothallium(III)-20-yl)-
3,5-dimethylphenyl]-2-[4-(5,10,15-triphenylporphinatothallium(III)-
20-yl)phenyl]ethyne (Tl2-M). Following a reported procedure,34
a
solution of Fb2-M (21.9 mg, 17.2 μmol) in CH2Cl2 (15 mL) was
treated with TlCl3·4H2O (329 mg, 860 μmol) in CH3OH (1.5 mL) at
room temperature for 6 h. Then the reaction mixture was diluted with
CH2Cl2 (25 mL) and washed with saturated aqueous NaHCO3 and
H2O. The organic layer was separated, dried (Na2SO4), filtered, and
concentrated to dryness under reduced pressure without any applied
heat. Subsequent chromatography [silica, CH2Cl2/hexanes (1:1)] gave
a green solid (18.7 mg, 62%): 1H NMR (300 MHz, CDCl3) δ 1.90 (s,
3H), 2.03 (s, 3H), 7.72−7.90 (m, 20H), 8.05−8.20 (m, 8H), 8.22 (d,
J(H−H)= 8.1 Hz, 1H), 8.34−8.48 (m, 7H), 8.88 (d, J(H−H) = 5.4
Hz, 1H), 9.03 (q, J(H−H) = 4.8 Hz, 2H), 9.07 (dd, J(H−H) = 4.8 Hz
and J(Tl−H) = 65.2 Hz, 8H), 9.08 (d, J(H−H) = 4.8 Hz, 1H), 9.10
(d, J(Tl−H) = 63.3 Hz, 2H), 9.25 (q, J(H−H) = 4.8 Hz, 2H);
MALDI-MS obsd 1755.1, calcd exact mass 1754.4; λabs (CH2Cl2) 436,
567, 608 nm.
1
solid (26 mg, 41%): H NMR (300 MHz, CDCl3) δ −2.74 (br, 2H),
−2.66 (br, 2H), 1.96 (s, 6H), 7.72−7.90 (m, 18H), 7.83 (s, 2H), 8.06
(d, J = 8.1 Hz, 2H), 8.18−8.26 (m, 12H), 8.29 (d, J = 8.1 Hz, 2H),
8.76 (d, J = 4.8 Hz, 2H), 8.82−8.88 (m, 10H), 8.89−8.96 (m, 4H);
MALDI-MS obsd 1278.4, calcd exact mass 1278.5; λabs (CH2Cl2) 421,
515, 549 nm.
Dichloro[1,2-bis[4-(5,10,15-triphenylporphinatothallium(III)-20-
yl)phenyl]ethyne] (Tl2-U). Following a copper-free procedure for
Sonogashira coupling of porphyrins,32,33 samples of Fb-U (23.7 mg,
36.4 μmol), Fb-U-I (27.0 mg, 36.4 μmol), and P(o-tol)3 (13.3 mg,
43.7 μmol) in toluene/triethylamine [(5:1), 15 mL) were degassed by
three freeze/pump/thaw cycles. Then Pd2(dba)3 (5.0 mg, 5.5 μmol)
was added, and the mixture was additionally degassed by three freeze/
pump/thaw cycles. The resulting mixture was stirred for 6 h at 40 °C
in a Schlenk line. The reaction mixture was allowed to cool and then
concentrated. The resulting crude product was chromatographed
(silica, CH2Cl2) to afford unreacted porphyrin monomers followed by
the desired dyad and then HMWM. The mixture of dyad and HMWM
was concentrated to dryness, dissolved in THF, and chromatographed
(SEC, THF) with gravity elution. The dyad-containing fractions were
combined and chromatographed [silica, CH2Cl2/hexanes (1:1)] to
afford 1,2-bis[4-(5,10,15-triphenylporphin-20-yl)phenyl]ethyne (Fb2-
U) as a purple solid (12.8 mg, 28%) with the following character-
ization data: MALDI-MS obsd 1250.9, calcd exact mass 1250.5; λabs
Chloro{5,10,15-triphenyl-20-[2,6-dimethyl-4-ethynylphenyl]-
porphinato}thallium(III) (Tl-H). Following a reported procedure,34
a
solution of Fb-H (53.3 mg, 80.0 μmol) in CH2Cl2 (66 mL) was
treated with TlCl3·4H2O (612 mg, 1.60 mmol) in CH3OH (3 mL) at
room temperature for 6 h. Then the reaction mixture was diluted with
CH2Cl2 (25 mL) and washed with saturated aqueous NaHCO3 and
H2O. The organic layer was separated, dried (Na2SO4), filtered, and
concentrated to dryness under reduced pressure without any applied
heat. Subsequent chromatography [silica, CH2Cl2/hexanes (1:1)] gave
a green solid (38.3 mg, 53%): 1H NMR (300 MHz, CDCl3) δ 1.79 (s,
3H), 1.92 (s, 3H), 3.27 (s, 1H), 7.66 (d, J(H−H) = 6.6 Hz, 2H),
7.72−7.78 (m, 3H), 7.80−7.88 (m, 6H), 8.06−8.18 (m, 3H), 8.32−
8.42 (m, 3H), 8.77 (d, J(H−H) = 4.8 Hz, 1H), 8.97 (d, J(H−H) = 4.8
Hz, 1H), 9.03 (d, J(Tl−H) = 63.6 Hz, 2H), 9.04 (d, J(Tl−H) = 66.0
Hz, 4H); MALDI-MS obsd 904.7; ESI-MS obsd 869.2359, calcd
869.2368 [({M − Cl}+ H)+, M = C48H32ClN4Tl]; λabs (CH2Cl2) 433,
565, 607 nm.
(CH2Cl2) 421, 516, 552 nm. Following a reported procedure,34
a
solution of Fb2-U (10.7 mg, 8.6 μmol) in CH2Cl2 (7 mL) was treated
with TlCl3·4H2O (431 mg, 431 μmol) in CH3OH (0.7 mL) at room
temperature for 6 h. Then the reaction mixture was diluted with
CH2Cl2 (25 mL) and washed with saturated aqueous NaHCO3 and
H2O. The organic layer was separated, dried (Na2SO4), filtered, and
concentrated to dryness under reduced pressure without any heat.
Subsequent chromatography [silica, CH2Cl2/hexanes (1:1)] gave a
Chloro{5,10,15-triphenyl-20-[4-ethynylphenyl]porphinato}-
thallium(III) (Tl-U). Following a reported procedure,34 a solution of
Fb-U (21.7 mg, 34.0 μmol) in CH2Cl2 (28 mL) was treated with
TlCl3·4H2O (260 mg, 680 μmol) in CH3OH (1.5 mL) at room
temperature for 6 h. Then the reaction mixture was diluted with
CH2Cl2 (25 mL) and washed with saturated aqueous NaHCO3 and
H2O. The organic layer was separated, dried (Na2SO4), filtered, and
I
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Article
green solid (7.6 mg, 51%): 1H NMR (300 MHz, CDCl3) δ 7.71−7.90
(m, 18H), 8.09 (d, J(H−H) = 7.8 Hz, 2H), 8.15 (d, J(H−H) = 6.6 Hz,
8H), 8.23 (d, J(H−H) = 7.8 Hz, 2H), 8.32−8.42 (m, 6H), 8.46 (d,
J(H−H) = 7.8 Hz, 2H), 9.03 (q, J(H−H) = 4.5 Hz, 4H), 9.08 (d,
J(Tl−H) = 65.1 Hz, 8H), 9.25 (q, J(H−H) = 4.8 Hz, 4H); MALDI-
MS obsd 1726.5, calcd exact mass 1726.3; λabs (CH2Cl2) 437, 567, 607
nm.
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ASSOCIATED CONTENT
■
S
* Supporting Information
Spectral data for all new compounds; EPR simulation
parameters for the dyads. This material is available free of
charge via the Internet at http://pubs.acs.org.
AUTHOR INFORMATION
■
Corresponding Author
*E-mail: holten@wustl.edu (D.H.), jlindsey@ncsu.edu (J.S.L.),
david.bocian@ucr.edu (D.F.B.).
Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS
■
This work was supported by a grant from the Chemical
Sciences, Geosciences and Biosciences Division, Office of Basic
Energy Sciences, of the U.S. Department of Energy to D.F.B.
(DE-FG02-05ER15660), J.S.L. (DE-FG02-96ER14632), and
D.H. (DE-FG02-05ER15661). Mass spectra were obtained at
the Mass Spectrometry Laboratory for Biotechnology at North
Carolina State University. Partial funding for the facility was
obtained from the North Carolina Biotechnology Center and
the National Science Foundation.
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